Missing thumbnails in gallery when I change the size of orig

Thanks in advance for your help!! I’m not sure if this is an Attachments issue, but I’ve noticed that when I change the size of an orig image in Media, it will change the filename of the thumbnail in my hosted image folder (so instead of simply image-120×120.jpg, it’s something weird like image-2842085352.jpg), therefore breaking the Thumbnail area filepath on the Gallery page (the “hover” magnifying glass is there instead). When I go back to Media and restore the original size of the image, the Thumbnail reappears on the Gallery page once again.

I noticed that *occasionally* but usually not, there is a small area on the right side of the “edit image” dialog box in the Media section that speaks to changing All instances of the image, just the Thumbnail, or All except the Thumbnail. I never see it before I go to resize the orig, but I’ve seen it appear a couple times AFTER I’ve already resized. I feel as though this sporadic appearance might have something to do with my Thumbnail saving issues?

Thanks again! I looked through this forum and couldn’t find anything, so I apologize if I missed something!
~Katie